SINGLE SIDEBAND MODULATORS
The present invention relates to the use of Single-Sideband (SSB) optical modulators, acting as electrically tuneable optical frequency shifters, to produce an optical reference comb (evenly or unevenly spaced) for use in low channel spacing High Density Wave Division Multiplexer (HDWDM) and coherent based systems. The use of SSB optical modulators is extended to the point where it is possible to eliminate (or reduce the number of) tuneable lasers required in such systems.
U.S. Pat. No. 5,265,112 uses SSB optical modulators to generate optical reference combs but does not cover the issue of how to uniquely identify one reference comb line from another. This is a serious issue in very narrow channel spacing systems with typical channel spacing of several GHz as there is the inherent risk of selecting an incorrect reference line. The present invention covers a technique which does allow a reference comb to be generated in which all the individual lines are uniquely identifiable.
Also is described the ability of SSB optical modulators to act as an electrically tuneable optical frequency shifter in firstly generating an even or unevenly spaced optical reference comb, with each reference line being uniquely identifiable, which can be used to stabilize the inter-channel separation between transmitters within multi-channel HDWDM or coherent based systems. Secondly, to replace the coherent transmitters' laser within localized and distributed multi-channel coherent based systems. Thirdly, to replace both the transmitter's and local oscillator's laser in both localized and distributed multi-channel coherent based systems.
GEC-Marconi Materials Technology Ltd--Caswell reported the first practical realization of an integrated optic Single-Sideband (SSB) optical modulator in 1993 which is shown in FIGS. 1a and 1b. The device, fabricated in GaAs, uses the concept described theoretically by Desormiere et al in "An Integrated Optical Frequency Translator for Microwave Frequency Operation" Journal of Lightwave Technology, No. 8 (1990) pp 506-513. The key element within the design is a Y-fed electro-optic directional coupler 1, whose electrode structure carries a travelling microwave signal. Since the coupler is excited 1 symmetrically by the Y-branch, in the absence of a modulating RF wave the optical distribution at the output of the coupler is also symmetric. If the two opposite halves of this distribution are sampled in output waveguides and then summed out of phase in a Mach-Zehnder interferometer section, the optical output of the device will accordingly be zero. This provides the carrier suppression feature of this device.
When a modulating signal RF is applied to the electrodes, mode-coupling can take place between the symmetric and antisymmetric modes. Since an output interferometer 2 is set to add opposite sides of the incoming light distribution in anti-phase, the antisymmetric components add to form a signal in an output waveguide. The signal frequency will be equal to that of the RF signal applied. Because of the carrier suppression feature the resulting output is a single frequency output equal in frequency to the carrier frequency shifted by a frequency equal to the RF modulation signal. Selecting whether the SSB modulator selects the Real or Image signal side allows positive or negative optical frequency shifters to be constructed.
The present invention relates to the uses of these SSB based optical frequency shifters within multi-channel optical systems highlighting their use in reducing, significantly, the number of lasers required. This method in eliminating the optical frequency control problems associated with all multi-channel coherent systems.
In multi-channel HDWDM or coherent based OFDM (Optical Frequency Division Multiplexed) systems there is the requirement to stabilize the inter-channel spacing between transmitters, avoiding cross-channel interference. Traditionally, Wavelength Management Units (WMU), consisting of an optical frequency scanning system, are used to scan the comb of wavelengths formed by combining the transmitters, measuring and hence controlling their inter-channel separation. Although every effort has been made to reduce the scan times, a compromise has to be made in the level of wavelength stability achievable, as the stability achievable is inversely proportional to the time interval between corrections. That is, shorter the interval is between scans, and hence correction, higher the repetition of frequency corrections and hence higher the optical frequency stability achieved. Also, in multi-channel systems, consisting of several 10's or 100's of channels, there is the practical problem of bringing into service these number of channels simultaneously, being able to identify each channel uniquely, and hence, the ability to determine the to unique frequency correction for each channel. Usually, this leaves the system designer to either start one or two channels up at a time (spaced sufficiently in optical frequency to allow unique identification) or to use multiple WMUs, each controlling a fraction of the total system. Even with multiple WMU systems start-up times could vary from several 10's of minutes to several hours.
Eliminating the WMU and placing the optical frequency control problem with the coherent transmitters themselves would seem to be an ideal solution as they would act independently and in parallel. Because each transmitter would be self controlling, the requirement to wait for `your turn` to be brought into service is eliminated resulting in system start-up times being limited only by the `slowest` transmitter (the transmitter having the largest optical frequency correction, i.e. the correction of start-up frequency to its allocated channel frequency). This would reduce system start-up times to several seconds not minutes or hours.
The practical way of implementing transmitter self frequency control is to tune the transmitter's laser to an optical wavelength reference, positioned in wavelength for the required channel using the same Automatic Frequency Control (AFC) approach as is used to lock the LO laser to one of transmitter's within the coherent receiver as described in U.S. Pat. No. 5,461,504. These optical wavelength references have been of interest for several years. Current techniques for generating suitable useable references include locking to a molecular absorption line of a gas, such as Acetylene, or the emission line of an ionized gas. Although, these techniques have been used to generate several 10's of useable reference wavelengths the large separation between features, usually 30-80 GHz, make their use restricted. Other techniques include modulating an FP laser or use of an SSB modulator in a circulating loop configuration as described in U.S. Pat. No. 5,265,112.
All of these types of comb generators suffer from the same problem--the inability to uniquely identify one reference line from another. This increases the risk of a transmitter locking to the wrong reference and hence sitting at the wrong channel. To avoid this situation supervisory system (possible based on a WMU) would be required to monitor and supervise the transmitters, checking for correct locking. The drawback is this would increase the start-up time again.